1. Field of the Invention
The present invention relates to an organic electroluminescent display device, and more particularly, to a top emission type organic electroluminescent display device having high luminance and a method of fabricating the same.
2. Discussion of the Related Art
In general, an organic electroluminescent display (OELD) device emits light by injecting electrons from a cathode and holes from an anode into an emission layer, combining the electrons with the holes, generating an exciton, and transforming the exciton from an excited state to a ground state. Unlike the liquid crystal display (LCD) device, the OELD device does not require an additional light source, and therefore, has the advantage of compact volume and light weight.
Since the OELD device has excellent characteristics, such as low power consumption, high luminance, high response time and light weight, the OELD device can be used in various electronic products, such as mobile phones, PDAs, camcorders and plam PCs. Moreover, because the OELD device is manufactured by a relatively simple fabricating process, the OELD has a low production cost as compared with the conventional LCD device.
OELD devices are categorized into a passive matrix type and an active matrix type, according to the driving method used. The passive matrix type OELD device has a simple structure and a more simple fabricating process. However, the passive matrix type OELD device requires high power consumption and is less conducive for use as a large size display. Moreover, as more electric lines are required in the device, the aperture ratio is reduced.
In contrast, the active matrix type OELD device has high emission efficiency and high quality images. FIG. 1 is a cross-sectional view showing the active matrix type OELD device according to the related art.
As shown in FIG. 1, the OELD device 10 includes first and second substrates 12 and 28 facing each other. The first substrate 12 is transparent and flexible. The first substrate 12 includes an array element 14 having a plurality of thin film transistors (TFTs) T and an organic electroluminescent diode E with a first electrode 16, an organic luminescent layer 18 and a second electrode 20. The organic luminescent layer 18 includes an organic material of one of a red, green and blue color deposited in each pixel region P.
The second substrate 28 includes a moisture absorbent powder 22. The moisture absorbent powder 22 is formed to remove moisture in the inner surface of the OELD device. The moisture absorbent powder 22 is disposed in a concave portion of the second substrate 28, and then, the moisture absorbent powder 22 is sealed by taping 25. A seal pattern 26 is disposed between the first and second substrates 12 and 28 such that the first and second substrates 12 and 28 are attached to each other.
In the above-mentioned structure, since the first electrode 16 is formed of a transparent metallic material, the light emitted from the organic luminescent layer 18 emits downward through the first electrode 16. Thus, this structure is often referred to as a bottom emission type.
FIG. 2 is a circuit diagram of an OELD device according to the related art.
As shown in FIG. 2, gate and data lines 42 and 44 are formed on the substrate 32. The gate and data lines 42 and 44 cross each other, and a switching element Ts is formed at each crossing of the gate and data lines 42 and 44. The switching element Ts includes a gate electrode 46, a source electrode 56 and a drain electrode 60. The gate electrode 46 is connected to the gate line 42. The source electrode 56 is connected to the data line 44 and separated from the drain electrode 60. A driving element TD is electrically connected to the switching element Ts. The driving element TD includes a gate electrode 68, a source electrode 66 and a drain electrode 63. The gate electrode 68 of the driving element TD is connected to the switching element Ts. The driving element TD is constituted by a p-type TFT, and a storage capacitor Cst is formed between the source and gate electrodes 66 and 68 of the driving element TD. The drain electrode 63 of the driving element TD is connected to the first electrode 16 (of FIG. 1) of the organic electroluminescent diode E. Moreover, a power line 55 is connected to the source electrode 66 of the driving element TD.
When a gate signal from the gate line 42 is supplied to the gate electrode 46 of the switching element Ts, a data signal from the data line 44 is supplied to the gate electrode 68 of the driving element TD through the switching element Ts. Then, the organic electroluminescent diode E is driven by the driving element TD such that the organic electroluminescent diode E emits light. In this case, a stored voltage in the storage capacitor Cst functions to maintain a voltage level in the gate electrode 68 of the driving element TD. Accordingly, even if the switching element Ts is in the off state, a voltage level in the organic electroluminescent diode E is kept. The switching element Ts and the driving element TD include a semiconductor layer of one of amorphous silicon and polycrystalline silicon. When the semiconductor is formed of amorphous silicon, the switching element Ts and the driving element TD can be easily fabricated.
FIG. 3 is a plane view showing an array element of an active matrix type OELD device according to the related art. As shown in FIG. 3, the active matrix type OELD device includes the switching element Ts, the driving element TD and the storage capacitor Cst in the pixel region P and on the substrate 32. According to characteristics of the OELD device, the pixel region P includes more than one switching element Ts and one driving element TD.
The gate line 42 and the data line 44 are formed on the substrate 32 with a gate insulating layer (not shown) interposed therebetween. The pixel region P is defined by crossings of the gate and data lines 42 and 44. The power line 55 is formed on the substrate 32. The power line 55 may be parallel to and separated from the gate line 42.
As mentioned above, the switching element Ts includes the gate electrode 46, the active layer 50 and the source and drain electrodes 56 and 60. The gate electrode 46 of the switching element Ts is connected to the gate line 42, and the source electrode 56 of the switching element Ts is connected to the data line 44. The drain electrode 60 of the switching element Ts is connected to the gate electrode 68 of the driving element TD through a gate contact hole 64.
The driving element TD includes the gate electrode 68, the active layer 62 and the source and drain electrodes 66 and 63. The source electrode 66 of the driving element TD is connected to the power line 55 through a power line contact hole 58. The drain electrode 63 of the driving element TD is connected to the first electrode 36 (FIG. 4). Moreover, the power line 55 and a silicon pattern 35 constitute the storage capacitor Cst with a gate insulating layer interposed therebetween. Further, the first electrode 36 of the organic electroluminescent diode E (of FIG. 1) is formed over the driving element TD. The first electrode 36 is connected to the drain electrode 63 of the driving element TD through the drain contact hole 65.
FIG. 4 is a cross-sectional view taken along the line IV-IV of FIG. 3.
As shown in FIG. 4, the driving element TD and the organic electroluminescent diode E are formed on the substrate 32 of the OELD device. The driving element TD includes the gate electrode 68, an active layer 62 and the source and drain electrodes 66 and 63. The organic electroluminescent diode E includes the first electrode 36, the organic luminescent layer 38 and the second electrode 40. The first electrode 36 contacts the drain electrode 63 of the driving element TD through the drain contact hole 65, and the organic luminescent layer 38 is interposed between the first and second electrodes 36 and 40. The first and second electrodes 36 and 40 function as anode and cathode, respectively. The storage capacitor Cst includes a silicon pattern 35 as a first storage electrode, a power line 55 as a second storage electrode, and a dielectric layer interposed therebetween. The second storage electrode 55 is connected the source electrode 66 of the driving element TD.
FIG. 5 is a schematic cross-sectional view of an organic electroluminescent diode according to the related art.
As shown in FIG. 5, the organic electroluminescent diode E including the first electrode 36, the organic luminescent layer 38 and the second electrode 40 is formed on the substrate 32. Though not shown, the substrate 32 includes the array element including the driving element TD (of FIG. 4). The first electrode 36 is connected to the driving element TD (of FIG. 4). The first and second electrode 36 and 40 function as an anode and a cathode, respectively. The organic luminescent layer 38 includes a hole injection layer (HIL) 38a, a hole transporting layer (HTL) 38b, an emitting material layer (EML) 38c, an electron transporting layer (ETL) 38d and an electron injection layer (EIL) 38e. Since an organic material may have a different mobility for the holes and than for the electrons, the HTL 38b and the ETL 38d will improve emitting efficiency. Moreover, the HIL 38a and EIL 38e will reduce an injection energy barrier of the hole and the electron.
Generally, the second electrode 40, as a cathode, is formed of a low work function material, such as calcium (Ca), aluminum (Al), magnesium (Mg), antigen (Ag) or lithium (Li). The first electrode 36, as an anode, is formed of a transparent metallic material, such as indium-tin-oxide (ITO) or indium-zinc-oxide (IZO). A transparent material is not used for the cathode (i.e., the second electrode 40). Since an ITO layer is deposited by a sputtering method, it is difficult to deposit the ITO layer on the organic luminescent layer 38 because of damage to the organic luminescent layer 38 by the sputtering process. Accordingly, the ITO layer, as an anode, is formed under the organic luminescent layer 38 such that the OELD device is a bottom emission type OELD device.
The bottom emission type OELD device according to the related art has a number of problems. First, the bottom emission type OELD has problems in luminance and aperture ratio. Second, since the anode is directly connected to the driving element, the driving element should include a p-type polycrystalline, which results in a complicated fabricating process for the OELD device. Moreover, when the anode is formed under the organic luminescent layer, the anode is exposed to air during the fabricating process such that an oxidation layer, which is formed from reaction between the anode and the air, causes a short circuit problem between the anode and the EIL.